In various applications, it is necessary for an operator to cause some remote apparatus to perform a function in accordance with a command given by the operator. For example, unmanned aircraft (drones), used as targets for the testing of guided missiles, must be flown throughout the same envelope as the intended targets, i.e., manned aircraft. This, of course, requires that the control inputs which would be supplied by an onboard pilot be supplied instead by telemetry from the ground. The missiles themselves may also be guided toward the target by signals from a ground station. In order that the drone's or missile's position always be known accurately, and in order to maintain control at all times, two or more radar stations may track the vehicle during its flight, one station of which also transmits the telemetry used for control. The radar station in control is called the command station and transmits, on the uplink channel, the command interrogation signal. The other radar station is the tracking station; its uplink transmission is called the track interrogation signal.
The command interrogation signal consists of a number of pulses, position-coded with such information as the address of the drone being controlled, the number of the data channel (for example, pitch channel), and the actual data being supplied on that channel. (Throughout the specification, "information" will refer to any intelligence transmitted by the pulses, whereas "data" will be used to designate only that portion of the information which contains an operator's command to the vehicle to perform a specific function, or such other information as the originator wishes to communicate to the addressee.) The track interrogation signal consists of the first and last pulses of the command interrogation format. In each case, the final pulse in the format is the range (or "main bang") pulse, whose return from the vehicle (whether by reflection or downlink) will be used by the radar station's receiver to determine the range to the drone.
In general, the number of separate items of information which can be conveyed by PPC in a single unsynchronized pulse train is necessarily limited to one less than the number of pulses. For example, in prior art 4-pulse command uplink formats, only three pieces of information (usually address, channel, and data) could be transmitted within the pulse train during any given pulse format repetition interval (PFRI). This imposed a limitation on the system in that any desired redundancy had to be provided by retransmitting some portion of the uplink information during the next pulse train.
Because the drone and its communication system operate in a relatively controlled environment, the noise affecting them is of a different character than would be encountered elsewhere. In the case of command interrogation, the signal power received at the vehicle can usually be made large enough, through transmitter input power and antenna gain, so that random noise is not a problem. The problem occurs when more than one drone, or a drone and the associated guided missile, are being controlled and tracked on the same range. The communication system for each vehicle must meet two stringent requirements: it must first lock on to the correct interrogation signal, and it must also reject all other interrogation signals. Failure of the system to meet the first requirement, viz, to lock onto any interrogation signal at all, results in the condition called "command loss" which, if it persists, must frequently be dealt with by destroying the vehicle for safety reasons. Similarly, if the communication system locks onto the wrong uplink, and the proper lock-on cannot be regained, the vehicle may have to be destroyed. The type of interference of most concern to the communication system engineer designing for an unmanned vehicle is pulse interference from nearby radars. Interfering pulses may either be random, that is, occurring at any time, or "synchronous," which is the word used to describe pulses which occur at or near the pulse format repetition frequency (PFRF), or one of its multiples, of the command interrogation signal. Of these two types of interference, the more important is the synchronous type.
Aside from the obvious expedients of changing the interfering radar's pulse repetition frequency (PRF), carrier frequency, and physical location, which are not always possible, the prior art of pulse communication in general has attempted to solve the interference problem through several methods. One such method, exemplified by Lockhart U.S. Pat. No. 3,987,447, is to predict, through various means, approximately when a pulse or a train of pulses will occur, to establish a "window" with a suitable tolerance around the period of predicted occurrence, and to ignore all pulses which do not arrive within that window. If there is a good deal of uncertainty about the expected arrival time of the pulse or pulse train, a larger window may be created; but a larger window will admit more noise. For the greatest amount of noise reduction, the smallest possible window is required, and this in turn requires great certainty about the arrival times of the pulses containing the desired data. The window method is especially suitable for communication systems such as radar stations, which have a fixed and relatively stable PRF. Of course, any noise which actually occurs during the window will be received along with the desired pulses. For random noise, if it is assumed that the noise pulses are equally likely to occur at any time and are independent of each other, the probability of error depends upon their average frequency (f) and the length (t) of the window, in accordance with Poisson's formula: ##EQU1## If the communication system is structured so as to reject any pulse train which includes such a random pulse, Equation (1) also gives the probability that a chosen pulse train will be rejected. For synchronous interference, which by definition is not distributed at all but instead constantly recurs at or about the communication system's PRF, the probability of error is unity of the synchronous pulse occurs during the window and zero otherwise. Once again, if the communication system is structured to reject pulse trains containing an interfering pulse, all pulse trains will be rejected. Thus, the Lockhart or window method alone is not suitable to remove synchronous interference.
Another rudimentary prior art method for avoiding pulse interference is the parity check, alluded to above. Essentially, the parity check assumes that if there is one and only one pulse received during an interval when one pulse is expected, the received pulse is the correct one. If some number of pulses other than one is received during the interval, the simple parity check is unable to determine which received pulse is correct, and consequently rejects all pulses. Once again, the smaller the interval during which parity is applied, the greater the probability of eliminating random noise and the greater the probability that the received pulse will be the desired one. Synchronous noise, however, will not be eliminated by the parity check alone. An additional disadvantage of relying on the parity check is its poor performance in the presence of multipath transmissions. For example, when the controlling transmitter is mobile, or when the target drone or missile is being controlled at low altitudes, the parity check causes rejection of almost all commands.
Communication reliability may also be enhanced, in the presence of noise, by making the channel update frequency (PRF divided by the number of channels, assuming once update per channel per PRF) several times greater than the rate at which information on the channels will have to be changed. For discrete channels, those which transmit commands for vehicle functions manifested by a limited number of physical states of the vehicle (such as speedbrake in/out), "smoothing" is performed by requiring that the same command be received a number of times before it is executed. The counterpart, on proportional channels, to discrete channel smoothing is detailed in Varsos U.S. Pat. No. 3,386,081. (Proportional channels deal with vehicle functions which may be continuous in nature, such as throttle position.) The Varsos concept selects, as the new valid data pulse, that pulse which is closest in position to the channel's last valid data pulse. Obviously, neither discrete channel smoothing nor the Varsos technique will eliminate synchronous noise.